MulteFire is a new radio access technology based on a Long Term Evolution (LTE), and may operate in a non-licensed frequency band without an aid of a licensed frequency band. The MulteFire expands the LTE into the non-licensed frequency band. A mechanism similar to a carrier sense technique in a Wireless Fidelity (WiFi) is introduced for a physical layer of the MulteFire, called Listen Before Talk (LBT), so as to fairly contend channel resources in the non-licensed frequency band with a WiFi device. In case that a Base Station (BS) or a User Equipment (UE) senses that a channel in the non-licensed frequency band is occupied, i.e. in case that the LBT fails, the BS or the UE stops transmitting a signal; in case that the BS or the UE senses that the channel is free, i.e., in case that the LBT succeeds, the BS or the UE transmits a signal.
In a LTE system, each downlink subframe includes a Cell Specific Reference Signal (CRS). If the CRS is not detected by the UF, the UE determines that a channel quality at this sample is poor. However, if a Discovery Reference Signal (DRS) is not detected in a MulteFire system, a reason thereof may be that the BS does not transmit the DRS in a DRS Transmission Window (DTxW) due to failure of the LBT; the reason thereof is probably also not the failure of the LBT, but that the UE does not detect the DRS due to a poor downlink channel quality. In case that the BS does not transmit the DRS due to the failure of the LBT in the DTxW, it cannot be determined that a channel quality of a subframe in which the DRS is not detected is poor. A Sample in which the DRS is not detected is a factor needing to be considered in in-sync detection or out-of-sync detection of a wireless link. In a related art, a Reference Signal Receiving Quality (RSRQ) of a reference signal of the Sample in which the DRS is not detected is considered to be low. This may incur that an assessed receiving signal quality in the in-sync detection or the out-of-sync detection of the UE is lower than an actual receiving signal quality. Additionally, the BS in the MulteFire system only transmits the CRS in a subframe in which the DRS exists or in another subframe including transmission of a Physical Downlink Shared Channel (PDSCH), but does not transmit the CRS in other subframes. Therefore, the UE may not use any subframe as the Sample for detecting a wireless link. A result of the in-sync detection or the out-of-sync detection of a wireless link in the MulteFire system using the method in the LTE system in the related art is not accurate.